Method of measuring liquid level and liquid-level gauge

ABSTRACT

For measurement of a liquid level, an optical fiber connected at its one end portion to force receiving means movably arranged in liquid to receive force from the liquid is dipped in the liquid together with the force receiving means, and a change in the force acting on said force receiving means when the liquid level changes is detected as a change in strain in the optical fiber by means of an optical fiber strain gauge connected to the other end of the optical fiber, thus measuring a water level of the liquid.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of measuring a heightof liquid level (water level) and a liquid-level gauge and moreparticularly, to a method and liquid-level gauge for measurement of awater level of liquid by measuring a change in force acted by liquid(buoyancy or hydrostatic pressure) on a body movably arranged in theliquid as a change in strain or strain level in an optical fiber.

[0002] Liquid-level gauges for measurement of a height of liquid level(water level) based on various principles have hitherto been proposed.For example, an electrostatic capacitance type liquid-level gauge(JP-A-2000-097750 or JP-A-11-030544), a barometric liquid-level gauge(JP-A-2000-088629), a float type liquid-level gauge (JP-A-10-148565 orJP-A-11-326015), an electrode type liquid-level gauge (JP-A-11-023346)and an electric wave type liquid-level gauge (JP-A-10-197617) have beenknown. Specifically, the present invention contemplates a float orcomparable type liquid-level gauge and a barometric or comparable typeliquid-level gauge, especially, using an optical fiber.

[0003] The float type liquid-level gauge detects a height of a floatthat ascends/descends as the liquid-level changes and conventionally, itis classified into two kinds of which one uses reed switches and amagnet and the other uses a wire or a tape. Structurally, the formerfloat type liquid-level gauge using reed switches and a magnet has manyreed switches that are operated by the magnet as the float ascends ordescends. On the other hand, in the latter float type liquid-level gaugeusing a wire, a measuring wire attached to a float is wound up tocalculate a liquid water level from a windup amount.

[0004] On the other hand, a barometric liquid-level gauge as exemplifiedin FIG. 20 has hitherto been employed. In the barometric liquid-levelgauge, an air supply pipe 300 having an open lower end is dippedvertically in liquid 200 stored in a tank 100 and compressed air 500 issupplied to the air supply pipe 300 by means of a pump 400. As thesupply of compressed air 500 to the air supply pipe 300 proceeds, airfills in the air supply pipe 300 in opposition to a pressure of theliquid 200 stored in the tank 100. When saturated in the air supply pipe300, the air is discharged to the liquid 200 in the form of bubblesthrough the lower open end of the air supply pipe 300. At that time, apressure P in the air supply pipe 300 equals a head pressure ρH when nogas pressure is applied onto the liquid level, the liquid level in thetank 100 is H and density of the liquid 200 is ρ. Therefore, thepressure P in the air supply pipe 300 is measured by means of a pressuresensor 700 and a measured value is indicated in terms of liquid levelheight on an indicator.

SUMMARY OF THE INVENTION

[0005] In the conventional float type liquid-level gauge, especially,using reed switches and a magnet, however, it is necessary that themagnet be built in the float and a great number of reed switched beincorporated in guide pipes for guiding the float, raising a problemthat the number of parts increases and the structure is complicated.

[0006] On the other hand, in the float type liquid-level gauge using awire, many parts such as a windup drum for the wire, a windup motor anda pulley are needed, so that the apparatus is increased in scale and isoften troubled because of mechanical windup, thus requiring laboriousand time-consuming work for repairs and maintenance.

[0007] Further, the conventional barometric liquid-level gauge facesproblems that the pump 400 for supplying the compressed air 500 to theair supply pipe 300 is needed and during measurement, the pump 400 mustbe driven constantly to supply the compressed air 500.

[0008] Under the circumstances, the present inventors have studied andconducted experiments in various ways by noticing a change in buoyancywhich a body receives from liquid as the liquid level changes in thefloat type or comparable type (suspension type) liquid-level gauge toconfirm that the water level of the liquid can be measured by detectingthe change in buoyancy as a change in strain in an optical fiber.

[0009] Experiments have been conducted also in the barometric orcomparable type liquid-level gauge to confirm that the liquid levelheight can be measured by displacing a pressure receiving member inaccordance with a change in liquid pressure, applying tension to anoptical fiber in accordance with the displacement to generate strain inthe optical fiber and detecting the strain.

[0010] The present invention has been made in the light of theconventional problems and the results of experiments and it is an objectof the invention to provide liquid-level measuring method andliquid-level gauge which can measure a water level accurately by usingan optical fiber connected to force receiving means movably arranged inliquid to receive force from the liquid and detecting a change in forcedue to a change in liquid level as strain in the optical fiber or achange in strain therein.

[0011] To accomplish the above object, in a method of measuring a liquidlevel according to the present invention, an optical fiber connected atits one end portion to force receiving means movably arranged in liquidto receive force from the liquid is dipped in the liquid together withthe force receiving means, and a change in the force acting on the forcereceiving means when the liquid level changes is detected as a change instrain in the optical fiber by means of an optical fiber strain gaugeconnected to the other end of the optical fiber.

[0012] The precedently determined correlation between changes in strainin the optical fiber and changes in liquid level of the liquid isconsulted on the basis of the detected value to determine a water levelof the liquid.

[0013] A liquid-level gauge according to the invention comprises anoptical fiber, force receiving means connected to one end portion of theoptical fiber and movably arranged, together with the optical fiber, inliquid to receive force from the liquid, and optical fiber strainmeasuring means connected to the other end portion of the optical fiberto detect, as a change in strain in the optical fiber, a change in theforce acting on the force receiving means when the liquid level of theliquid changes.

[0014] Preferably, the optical fiber strain measuring means is aBrillouin-optical time domain reflector (hereinafter simply referred toas B-OTDR).

[0015] According to one aspect of the invention, in a method ofmeasuring a liquid level, a float having a cross-sectional form that isuniform in the height direction and a specific weight value less thanthat of liquid is dipped in the liquid, the float is supported by anoptical fiber in such a manner that an upper end of the optical fiberconstantly protrudes from the liquid level, the optical fiber isconnected at its upper end to an optical fiber strain gauge, and achange in buoyancy acting on the float as the water level of the liquidchanges is detected as a change in strain in the optical fiber by meansof the optical fiber strain gauge, thus measuring a water level of theliquid.

[0016] According to a second aspect of the invention, in a method ofmeasuring a liquid level, a suspension member having a cross-sectionalform that is uniform in the height direction and a specific weight valuenot less than that of liquid is suspended by an optical fiber so as tobe dipped in the liquid in such a manner that an upper end of thesuspension member constantly protrudes from the liquid level, theoptical fiber is connected to an optical fiber strain gauge, and achange in buoyancy acting on the suspension member as the water level ofthe liquid changes is detected as a change in strain in the opticalfiber by means of the optical fiber strain gauge, thus measuring a waterlevel of the liquid.

[0017] In embodiments of the float type liquid-level gauge according tothe invention, a liquid-level gauge comprises a float having across-sectional form that is uniform in the height direction and aspecific weight value less than that of liquid and dipped in the liquid,an optical fiber for supporting the float in such a manner that an upperend of the float constantly protrudes from the liquid level, and anoptical fiber stain gauge for detecting a change in buoyancy acting onthe float due to a change in water level of the liquid as a change instrain in the optical fiber.

[0018] A liquid-level gauge comprises a suspension member having across-sectional form that is uniform in the height direction and aspecific weight value not less than that of liquid, an optical fiber fordipping the suspension member in the liquid while suspending thesuspension member in such a manner that an upper end of the suspensionmember constantly protrudes from the liquid level, and an optical fiberstrain gauge for detecting a change in buoyancy acting on the suspensionmember due to a change in water level of the liquid as a change instrain in the optical fiber.

[0019] In the liquid-level gauge of the present invention, as the liquidlevel changes, the magnitude of buoyancy acting on the float orsuspension member by the liquid changes. Since the cross-sectional areaof each of the float and the suspension member is uniform in thelongitudinal direction, the magnitude of the change in liquid level isaccurately proportional to the change in buoyancy acting on the float orsuspension number. Also, the change in strain level in the optical fiberis accurately proportional to the change in buoyancy. In the case of thefloat, as the liquid water level increases, tension applied to theoptical fiber increases in proportion to the increased water level toraise the strain. In the case of the suspension member, as the waterlevel of the liquid increases to increase the buoyancy, tension appliedto the optical fiber decreases in inverse proportion to an increase inwater level and the strain decreases correspondingly. Accordingly, ineither case, when the change in buoyancy as the change in strain causedin the optical fiber is detected by means of the optical fiber straingauge, the water level of the liquid can be measured from thecorrelation between changes in liquid water level and changes in strain.

[0020] The liquid to be measured is in no way limited to water in thepresent invention but the invention may also be applied to measurementof the liquid level of various liquids such as oil and medicines and itwill be appreciated that “water level” will be used as a broad wordmeaning the liquid level height of these kinds of liquids.

[0021] According to a third aspect of the invention, in a method ofmeasuring a liquid level by generating strain in an optical fiber inaccordance with liquid pressure and detecting the strain to measure aheight of liquid level, portions of the optical fiber dipped in liquidare fixed to a fixing member and a pressure receiving member provided ina pressure receiver, respectively, tension is applied to a fiber portionbetween the fixing member and the pressure receiving member to generatestrain in the optical fiber when the pressure receiving member isdisplaced by a liquid pressure, and the strain is detected by means ofan optical fiber strain gauge.

[0022] In embodiments of the barometric liquid-level gauge according tothe invention, a liquid-level gauge comprises an elongated opticalfiber, a pressure receiver having a pressure receiving memberdisplaceable by liquid pressure, fixing members for fixing the opticalfiber, and an optical fiber strain gauge for detecting strain in theoptical fiber, portions of the optical fiber dipped in the liquid beingfixed to the pressure receiving member and the fixing member,respectively, in the liquid and one end of the optical fiber beingconnected to the optical fiber strain gauge.

[0023] In the liquid-level gauge, the pressure receiver has apressure-tight vessel main body and the pressure receiving member is apiston movable over an opening of the vessel main body to cover ithermetically.

[0024] In the liquid-level gauge, the pressure receiver has apressure-tight vessel main body and the pressure receiving member is abellows having a pressure receiving plate to hermetically cover anopening of the vessel main body.

[0025] In the above barometric type of the present invention, as theliquid level height changes to change the liquid pressure, the magnitudeof pressure acting on the pressure receiving member dipped or immersedin the liquid also changes to displace (move or deform) the pressurereceiving member. The magnitude of a change in liquid level height isproportional to the magnitude of force resulting from multiplying achange in pressure at a position where the pressure receiver is placedby a pressure receiving area of the pressure receiving member appliedwith the pressure change. When the pressure receiving member isdisplaced by a liquid pressure, the optical fiber deforms in proportionto a displacement of the pressure receiving member. In other words, whenthe liquid level height increases to increase the liquid pressureapplied to the pressure receiving member, tension applied to the opticalfiber increases in proportion to an increased liquid pressure toincrease strain. On the other hand, when the liquid level heightdecreases to decrease the liquid pressure applied to the pressurereceiving member, tension caused in the optical fiber decreases inproportion to a decreased liquid pressure to decrease strain.Accordingly, when the correlation between changes in liquid level heightand changes in strain is determined in advance, by detecting a change inliquid pressure acting on the pressure receiving member as a change instrain generated in the optical fiber by means of the optical fiberstrain gauge, a liquid level height can be measured accurately from thecorrelation between the liquid level height and the strain.

[0026] In the liquid-level gauge, the vessel main body is provided witha stopper for limiting movement of the pressure receiving member to apredetermined range.

[0027] In the liquid-level gauge, the optical fiber has a plurality ofportions spaced apart from each other in a direction of depth of theliquid and each fixed by the pressure receiving member and the fixingmember.

[0028] In the aforementioned barometric type of the invention, thestopper limits the movement of the pressure receiving member to preventthe optical fiber from being broken under the application of excessivetension when the liquid pressure increases.

[0029] With a plurality of portions of optical fiber spaced apart fromeach other in the liquid depth direction and each fixed by the pressurereceiving member and fixing member, a liquid level height from aposition of each portion can be measured in each portion. For example,when three portions fixed by pressure receiving members and fixingmembers are provided, a lowermost portion is in charge of measurement ofa low liquid level, an intermediate portion is in charge of measurementof an intermediate liquid level and an uppermost portion is in charge ofmeasurement of a high liquid level. In other words, the lowermostportion is for measuring a depth between its position and theintermediate portion, the intermediate portion is for measuring a depthbetween its position and the uppermost portion and the uppermost portionis for measuring a depth between its position and the liquid level.Accordingly, in case, for example, the intermediate portion is dipped inthe liquid during measurement, a depth between the liquid level and theintermediate portion can be measured by detecting strain caused in theintermediate portion and when this depth is added with a depth betweenthe lowermost portion and the intermediate portion, an actual liquidlevel height can be determined. In case the uppermost portion is dipped,a depth between the liquid level and the uppermost portion can bemeasured by detecting strain caused in the uppermost portion and whenthis depth is added with the depth between the lowermost portion and theintermediate portion and the depth between the intermediate portion andthe uppermost portion, an actual liquid level height can be determined.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a sectional diagram showing a liquid-level gaugeaccording to a first embodiment of the invention.

[0031]FIG. 2 is a sectional view on line II-II of FIG. 1.

[0032]FIG. 3 is a diagram showing at sections (a), (b) and (c)modifications of the sleeve in the first embodiment.

[0033]FIG. 4 is a diagram showing a liquid-level gauge according to amodified embodiment of the invention.

[0034]FIG. 5 is a diagram showing at sections (a) and (b) a modificationof the float.

[0035]FIG. 6 is a block diagram showing the fundamental construction ofan optical fiber strain gauge.

[0036]FIG. 7 is a graph showing the relation between a change in strainand a change in water level in the first embodiment.

[0037]FIG. 8 is a sectional diagram showing a liquid-level gaugeaccording to a second embodiment of the invention.

[0038]FIG. 9 is a sectional view on line IX-IX of FIG. 8.

[0039]FIG. 10 is a graph showing the relation between a change in strainand a change in water level in the second embodiment.

[0040]FIG. 11 is a diagram showing at sections (a) and (b) a front viewof a liquid-level gauge according to a third embodiment of the inventionand support of a float by an optical fiber.

[0041]FIG. 12 is a sectional diagram showing a liquid-level gaugeaccording to a fourth embodiment of the invention.

[0042]FIG. 13 is a sectional view of a pressure receiver in the fourthembodiment.

[0043]FIG. 14 is a sectional diagram showing a liquid-level gaugeaccording to a fifth embodiment of the invention.

[0044]FIG. 15 is a diagram showing a liquid-level gauge according to asixth embodiment of the invention.

[0045]FIG. 16 is a sectional view of a pressure receiver in the sixthembodiment.

[0046]FIG. 17 is a sectional diagram showing a liquid-level gaugeaccording to a seventh embodiment of the invention.

[0047]FIG. 18 is a sectional diagram showing a liquid-level gaugeaccording to an eighth embodiment of the invention.

[0048]FIG. 19 is a sectional diagram showing a liquid-level gaugeaccording to a ninth embodiment of the invention.

[0049]FIG. 20 is a schematic diagram showing an example of conventionalbarometric liquid-level gauge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] Embodiments of the invention will now be described with referenceto the accompanying drawings.

[0051] Referring first to FIGS. 1 and 2, an embodiment of a float typeliquid-level gauge according to a first embodiment of the invention willbe described. In these figures, a vessel 1 stores liquid 2 representingan object to be measured, a sleeve 3 is arranged in the vessel 1, afloat 4 is arranged in the sleeve 3, an optical fiber 5 supports thefloat 4 to keep it at a constant height, and an optical fiber straingauge 6 as represented by a Brillouin-optical time domain reflectometer(B-OTDR) detects a strain level in the optical fiber 5. The float 4,optical fiber 5 and B-OTDR 6 constitute float type liquid-level gaugemeans generally designated by reference numeral 7. More generally, thefloat constitutes force receiving means movably arranged in liquid toreceive force from the liquid.

[0052] The sleeve 3 takes a cylindrical form having its top endprotruding upwardly of the vessel 1 and a plurality of liquid inletholes 11 formed in its periphery in the height direction or longitudinaldirection at intervals of suitable distance. The number and size of theliquid inlet holes 11 can be changed suitably and are not specified inparticular. An upper lid 13 having a small hole 12 through which theoptical fiber 5 passes is fixedly fitted in an upper opening of thesleeve 3 and a bottom plate 14 is fixed to an inner wall near the bottomend. A small hole 15 is formed in the bottom plate 15 in the center anda fixing pipe 16 for fixing the optical fiber 5 is fitted in the smallhole 15.

[0053] The sleeve 3 is made of any material that cannot be eroded by theliquid 2, such as plastics, metal, ceramics, wood or the like. Thesleeve 3 can take any form other than the cylindrical one, for example,a hurdle form, a blind form or a net form as shown at (a), (b) and (c)in FIG. 3 having a structure that permits the liquid 2 to passtherethrough. Preferably, to facilitate the passage of the optical fiber5, each of the sleeve 3, upper lid 13 and bottom plate 14 isstructurally divided into halves and after the optical fiber 5 is passedthrough the small hole 12 and fixing pipe 16, these halves are puttogether and bonded to each other by bonding agent or fixed to eachother mechanically by means of screws.

[0054] In the case of the float type liquid-level gauge 7, the sleeve 3is mainly used to prevent upside-down motion of the float 4 when aliquid level 17 of liquid 2 lowers until most of the float 4 projectsfrom the liquid level 17. Therefore, the sleeve 3 is not limited to thecylindrical form that accommodates the whole of the float 4 but forexample, may take such a structure as shown in FIG. 4 that cylindricalguide members 18 and 19 of a suitable length are fixed to roof plate 1 aand bottom plate 1 b of a hermetic vessel 1′, respectively, upper andlower ends of the float 4 are loosely inserted in these guide members 18and 19, respectively, so as to be prevented from undergoing upside-downmotion. Accordingly, the upper lid 13 and bottom plate 14 of the sleeve3 are not always necessary.

[0055] Referring to FIG. 2 showing a section on line II-II of FIG. 1,the float 4 is constructed of three round bars 4A to 4C so bundled as tocontact with each other, leaving a triangular hollow cavity 21 in thecenter (FIG. 1) through which the optical fiber 5 passes. Each of theround bars 4A to 4C is made of a material having a specific weight valueless than that of the liquid 2 such as wood or plastics. Fixedly fittedin the upper and lower ends of the hollow cavity 21 are pipes 22 and 23through which the optical fiber 5 passes. The float 4 has a lengthsufficient to cover a change range of the liquid level 17 of liquid 2and is dipped in the liquid 2 in such a manner that its upper endprojects from the maximum water level of the liquid level 17.

[0056] In the first embodiment, the float 4 is exemplified as beingformed of the three round bars 4A to 4C but this is not limitative andthe float may alternatively be formed of a single round bar having acentral through-hole for passage of the optical fiber 5. The circularsectional form is not limitative and the float may take a desiredsectional form, for example, a polygonal form such as rectangular ortriangular form. In short, any form can be taken provided that thesectional form is uniform over the whole length. For example, as shownat (a) in FIG. 5, the float can be formed into a squared pillar by twosquared bars 4D and 4E and two plates 4F and 4G or as shown at (b) inFIG. 5, it can be formed into a squared pillar by two squared bars 4Dand 4E and two spacers 4H interposed between upper ends and betweenlower ends of the squared bars 4D and 4E.

[0057] Preferably, since the optical fiber 5 passes through the centerof the float 4, the float 4 and each of the pipes 22 and 23 can bestructurally divided into halves and after the passage of the opticalfiber 5, these halves can be put together and jointed integrally bybonding agent or fixed to each other mechanically by means of, forexample, screws. Especially, when these halves are attached withconnectors, reinforcement sleeves for fusion connection, the attachedmembers can be utilized for mechanical fixing and screws can bedispensed to facilitate connection.

[0058] The optical fiber 5 is sufficiently longer than the float 4 andis passed through the small hole 12 in the upper lid 13, pipe 22, hollowcavity 21, pipe 23 and fixing pipe 16 in the bottom plate 14 so as topass through the sleeve 3 and float 4. A surplus lower end portion 5 ahaving a length of 2 m or more is accommodated in a space 25 under thebottom plate 14 and its terminal end is applied with a reflectionpreventive process by coating silicon oil. The optical fiber 5 has itsupper end connected to the B-OTDR 6, two intermediate portions fixed tothe fixing pipes 16 and 22 by, for example, bonding agent and a fiberportion 5A between the two pipes 16 and 22 that forms a tension changedetecting region. It is important to fix the two intermediate portionsto the fixing pipes 16 and 22 without applying tension to the opticalfiber 5. It is desired that the optical fiber 5 should pass through thesmall hole 12, pipe 23 and hollow cavity 21 without contacting theirinner walls. In the liquid-level gauge shown in FIG. 4, the lower endportion of the optical fiber 5 is fixed to the bottom plate 1 b ofvessel 1′ and a surplus portion 5 a protrudes downwardly of the vessel1′.

[0059] The optical fiber 5 is not limited to a solid wire but may be anoptical fiber tape, an optical fiber cord or another type usedintegrally with a member for protection and reinforcement of the opticalfiber 5.

[0060] The B-OTDR 6 is a unit for measuring strain distribution or lossdistribution in the longitudinal direction of the optical fiber bydetecting and analyzing natural Brillouin scattering light, back Raleighscattering light and Brillouin amplified light in the optical fiber andits fundamental construction is shown in FIG. 6.

[0061] The Brillouin scattering light is one of scattering light raysgenerated when light travels through a medium (optical fiber). When thelight is scattered, it is shifted relative to incident light by afrequency inherent to the medium of the optical fiber and in thepresence of changes in strain or temperature, the shift amount changesin proportion to the strain in the optical fiber or temperature.Accordingly, by detecting an amount of change in the Brillouin frequencyshift, strain applied to the optical fiber can be measured continuouslyin the longitudinal direction.

[0062] Since an amount of change in frequency shift due to temperaturechange is very smaller than the change amount due to strain change(0.002%/° C.), the influence of temperature can be neglected when thetemperature change is small (about 5° C.) during measurement of theamount of change in Brillouin frequency shift due to strain.

[0063] In measurement, continuous light of narrow spectrum line widthemitted from a coherent light source 30 is first branched to signallight 32 and reference light 33 by means of an optical directionalcoupler 31. The signal light 32 is changed in light frequency stepwiseon time series base and converted into a light pulse train having a timewidth of about 2 μsec (light frequency conversion) by means of a lightfrequency converter 34, and further converted into a light pulse 36having a time width of about 10 nsec to 1 μsec by means of a light pulsemodulator 35 and thereafter, caused to be incident on an optical fiver 5via a light directional coupler 37. The light pulse 36 coming into theoptical fiber 5 undergoes Raleigh scattering and Brillouin scattering inthe optical fiber 5 to generate back scattering light 38. The backscattering light 38 is caused to be incident on a coherent lightreceiver 39 via the light directional coupler 37. On the other hand, thereference light 33 is also incident on the light receiver 39 and the twolight rays are subjected to a signal processing to detect a change instrain in the optical fiber 5. In this case, the intensity of the backscattering light 38 in Brillouin scattering is so weak as to amount toabout {fraction (1/100)} of that of the Raleigh scattering light but byadopting the coherent detection technique and light frequency conversiontechnique, the Brillouin scattering light in the optical fiber 5 can bedetected with high accuracy. This type of B-OTDR for measurement ofstrain in the optical fiber has hitherto been known (for example,JP-A-10-90121, JP-A-9-89714, JP-A-5-231923 and JP-A-10-197298) and acommercially available one can be used.

[0064] In the float type liquid-level gauge 7 as above, the float 4dipped in the liquid 2 receives vertically upward force, that is,buoyancy from the liquid 2. The buoyancy equals a weight value of theliquid 2 excluded by the float 4. Consequently, the optical fiber 5supporting the float 4 is applied with tension and strain is generated.As the water level of the liquid 2 changes, the magnitude of buoyancyacting on the float 4 changes, with the result that tension applied tothe optical fiber 5 and the strain generated in the fiber also change inproportion to the magnitude of buoyancy. Since the cross-sectional areaof the float 4 (also the optical fiber 5) is uniform in the longitudinaldirection, the amount of change in water level of the liquid isaccurately proportional to a change in buoyancy acting on the float 4.Also, a change in tension and a change in strain in the optical fiber 5are also accurately proportional to the change in buoyancy. Then, thechange in strain is measured by means of the B-OTDR 6. When changeamounts of strain and changes in water level are measured in advancethrough experiments and the correlative relation between them isdetermined, a water level of the measured liquid or an amount of changein water level can be measured accurately by measuring a change instrain.

[0065] Referring to FIG. 7, measurement results of strain changes andwater level changes are illustrated graphically. For convenience ofexperiments, the upper limit of water level change is set to 700 mm butby changing design conditions, a larger change in water level can bemeasured.

[0066] The change amount of water level is related to the change amountof strain in the optical fiber 5 by equation (1). $\begin{matrix}{{{\left( {\Delta \quad {L/{\Delta ɛ}}} \right) = {4{{EeqAs}/\left( {S\quad \rho \quad g} \right)}}}\quad {{{where}\quad A_{S}} = \left( {A_{F} + A_{C}} \right)}\quad \quad {{Eeq} = {\left( {{A_{F}E_{F}} + {A_{C}E_{C}}} \right)/A_{S}}}}\quad} & (1)\end{matrix}$

[0067] ΔL (m): change amount of liquid level

[0068] Δε (%): change amount of strain in optical fiber

[0069] A_(F) (m²): cross-sectional area of optical fiber

[0070] E_(F) (GN/m²): modulus of elasticity of optical fiber

[0071] A_(C) (m²): cross-sectional area of protective layer of opticalfiber

[0072] E_(C)(GN/m²): modulus of elasticity of protective layer

[0073] S(m²): cross-sectional area of float

[0074] L₀ (m): length of float

[0075] ρ (1000 kg/m³): density of liquid

[0076] g (9.8 m/sec²): acceleration of gravity

[0077] Accordingly, the geometrical dimension of the float 4 can bedetermined by consulting the above equation (1).

[0078] Referring to FIGS. 8 and 9, a liquid-level gauge according to asecond embodiment of the invention will be described.

[0079] In these figures, constituent members identical to those in FIGS.1 and 2 are designated by identical reference numerals and will notalways be described for avoidance of prolixity of explanation. In thesecond embodiment, a suspension member 40 having a specific weight valuenot less than that of liquid 2 is suspended by an optical fiber 5 so asto be dipped in the liquid 2 in such a manner that its upper endconstantly protrudes from the liquid level. This type of liquid-levelgauge is called herein a suspension type liquid-level gauge 7′.

[0080] The suspension member 40 is constructed quite identically to theaforementioned float 4 with the only exception that the suspensionmember 40 is made of a material having a specific weight value not lessthan that of the liquid 2.

[0081] The optical fiber 5 differs from the optical fiber 5 used in theaforementioned float type liquid-level gauge 7 in that two lowerportions of the former optical fiber are respectively fixed to a pipe 41fitted in a small hole 12 in a top lid 13 of sleeve 3 and a pipe 23mounted to a lower end portion of the suspension member 40 by bondingagent and the present optical fiber passes through a pipe 22 and a smallhole 15 in a bottom plate 14 without contacting them. A fiber portion 5Abetween the pipes 22 and 23 forms a tension change detecting region.More generally, the suspension member constitutes force receiving meansmovably arranged in the liquid to receive force from the liquid.

[0082] In the suspension type liquid-level gauge 7′ as above, thesuspension member 40 suspended by the optical fiber or optical fiber 5is dipped in the liquid 2. In the optical fiber 5 suspending thesuspension member 40, tension is generated to cause strain to takeplace. When dipped in the liquid 2, the suspension member 40 receivesbuoyancy from the liquid 2. The buoyancy equals a weight value of theliquid 2 excluded by the suspension member 40. Because of this buoyancy,the tension in the optical fiber 5 suspending the suspension member 40decreases to decrease the strain. As the water level of the liquid 2changes, the magnitude of the buoyancy acting on the suspension member40 changes and as a result, the tension applied to the optical fiber 5and the strain generated in the fiber change in proportion to themagnitude of the buoyancy. Since the cross-sectional area of thesuspension member 40 is uniform in the longitudinal direction, themagnitude of a change in water level is accurately proportional to achange in buoyancy acting on the suspension member 40. A change intension and a change in strain are accurately proportional to the changein buoyancy. Then, this change in strain is measured by means of theB-OTDR 6. Strain change amounts and liquid water level changes aremeasured in advance through experiments to determine the correlationbetween them, so that by measuring a change in strain, a water level ofthe liquid or an amount of change in water level can be measuredaccurately. In the case of the suspension type liquid-level gauge, too,geometrical dimensions of the suspension member 40 can be determined byconsulting the aforementioned equation (1).

[0083] Referring to FIG. 10, measurement results of strain changes andwater level changes of the liquid in the suspension type liquid-levelgauge 7′ are graphically shown. In contrast to the float typeliquid-level gauge 7, the strain increases in inverse proportion to adecrease in water level of the liquid level in the case of thesuspension type liquid-level gauge 7′. For the convenience ofexperiments, the upper limit of the water level change is set to 600mmbut a larger change in water level can be measured by changing designconditions.

[0084] Referring to FIG. 11, a liquid-level gauge according to a thirdembodiment of the invention will be described. FIG. 11 illustrates atsection (a) a front view of the third embodiment and at section (b)supporting of floats by an optical fiber.

[0085] In the present embodiment, three float type liquid-level gauges7A, 7B and 7C are mounted to a pillar 51 standing upright on a supportbase 50 at different heights in a partly overlapping fashion. Therespective liquid-level gauges 7A, 7B and 7C are connected in series bya single optical fiber 5. Each of the liquid-level gauges 7A, 7B and 7Cis quite identical to the liquid-level gauge 7 shown in FIG. 1. Theoptical fiber 5 is fixed to the float 4 at point A and to the pillar 51at point B and a fiber portion 5A between the points A and B in eachfloat 4 is a tension change detecting region. During measurement, theliquid-level gauge 7A is in charge of a high water level, theliquid-level gauge 7B is in charge of a medium water level and theliquid-level gauge 7C is in charge of a low water level.

[0086] With this construction, the respective liquid-level gauges 7A, 7Band 7C share in the range of water level to be measured for making therespective actual measurement ranges narrow, thus increasing theefficiency of measurement.

[0087] In the foregoing embodiments, the liquid-level gauge has beendescribed by way of example of one for measuring the water level ofliquid in the vessel but this is not limitative and it may also be usedfor measurement of the water level in rivers and storing reservoirs bypermitting water to freely come into or go out of the vessel. Inaddition, by using a material that is not eroded by oil and medicines,the oil level in an oil reserving tank and the liquid level of medicinescan also be measured.

[0088] In the third embodiment shown in FIG. 11, an example using threefloat type liquid-level gauges 7A, 7B and 7C has been described but asingle liquid-level gauge can also be constructed by using a suitablenumber of float type liquid-level gauges 7 shown in FIG. 1 andsuspension type liquid-level gauges 7′ shown in FIG. 8 in combination orusing a plurality of suspension type liquid-level gauges 7′ alone.

[0089] As described above, in the liquid-level measuring method andliquid-level gauge according to the invention, the float having itscross-sectional shape that is uniform in the height direction and aspecific weight value less than that of liquid is dipped in the liquid,the float is supported by the optical fiber in such a manner that itstop end constantly protrudes from the liquid level, the optical fiber isconnected to the optical fiber stain gauge, and a change in buoyancyacting on the float due to a change in water level of the liquid isdetected as a change in strain in the optical fiber by means of theoptical fiber strain gauge to measure a water level of the liquid,whereby the construction of the float can be simplified, the number ofparts can be reduced and a change in liquid level can be measuredaccurately. Further, occurrence of troubles can be suppressed tofacilitate the maintenance and especially, electric parts and a magnetneed not be built in the float, thus permitting applications toflammable or non-conductive fluid.

[0090] Referring now to FIGS. 12 to 19, embodiments of a barometricliquid-level gauge according to the invention will be described. Abarometric liquid-level gauge according to a fourth embodiment of theinvention will first be described with reference to FIGS. 12 and 13. Inthe figures, an open type tank 100 stores liquid 200 representing anobject to be measured, and a liquid level gauge 110 measures a height(liquid level height) of water surface 200 a of the liquid 200.

[0091] The liquid-level gauge 110 comprises an optical fiber 120 havingone end 120 a positioned externally of the tank 100 and the other enddipped or immersed in the liquid 200, a support base 130 disposed on thebottom of the tank 100, a fixing member 140 and a pressure receiver 150which are arranged on the support base 130, an optical fiber straingauge 160 as represented by a B-OTDR 160 arranged on the tank 100, and afixing member 170 attached to a pressure receiving member 150B of thepressure receiver 150.

[0092] One end 120 a of the optical fiber 120 is connected to the B-OTDRand the other end 120 b thereof is coated with silicon oil so as to beapplied with a reflection preventive treatment. A suitable intermediateportion of the optical fiber 120 close to the other end and immersed inthe liquid 200 has a partial portion of suitable length (between pointsA and B) that passes through fixing pipes 180 and 190 attached to theaforementioned fixing members 140 and 170, respectively. The partialportion is fixedly secured to these fixing pipes by bonding agent 200 toform a strain detecting portion 120A. The optical fiber 120 inserted inthe respective pipes 180 and 190 is fixed thereto in such a way that theoptical fiber is not slackened to prevent the strain detecting portion120A from being deformed and applied with initial tension. A portion ofoptical fiber 120 extending from the fixing member 170 and terminatingin a terminal end 120 b forms a surplus length portion 120B.

[0093] The fixing member 140 stands upright on one end portion of thesupport base 130, having an upper end to which one end of the straindetecting portion 120A of optical fiber 120 is fixed through the mediumof the fixing pipe 180.

[0094] As shown in FIG. 13, the pressure receiver 150 includes apressure-tight vessel main body 150A constituting a cylinder, and thepressure receiving member 150B hermetically blocking an opening 220 ofthe vessel main body 150A, thus forming an airtight vessel havinginternal pressure equal to atmospheric pressure. The vessel main body150A takes the form of a sufficiently rigid cylinder of constant innerdiameter having its one end opened and the opening 220 is disposed atthe end portion on the support base 130 so as to oppose the fixingmember 140. A stopper 240 is disposed in the vessel main body 150A tolimit movement of the pressure receiving member 150B to a predeterminedrange, thereby preventing the optical fiber 120 from being broken.

[0095] The pressure receiving member 150B includes a piston 250 ofcircular plate form movably accommodated in the vessel main body 150Aand an O-ring 260 snugly fitted in a circular groove formed in the outerperiphery of the piston 250. A surface of piston 250 exposing to theoutside through the opening 220 forms a pressure receiving surface 250 afor receiving a liquid pressure P. The fixing member 170 has one endconnected to the pressure receiving surface 250 a and its tip end towhich the other end B of strain detecting portion 120A of the opticalfiber 120 is fixed through the fixing pipe 190.

[0096] Next, the measurement principle on which the liquid-level gauge110 according to the fourth embodiment is based will be described withreference to FIG. 12.

[0097] Firstly, the optical fiber 120 is immersed in the liquid 200,together with the support base 130, fixing members 140 and 170 andpressure receiver 150. With the pressure receiver 150 immersed in theliquid 200, the piston 250 is applied with the liquid pressure at itspressure receiving surface 250 a, so that the piston 250 moves towardthe vessel main body 150A until the liquid pressure P balances with theatmospheric pressure in the vessel main body 150A, thus expanding thedistance between the fixing members 140 and 170. As a result, tension isapplied to the strain detecting portion 120A of the optical fiber 120,causing strain. The strain changes in proportion to a change in liquidpressure P. In other words, when the amount of liquid 200 in the tank100 increases to raise the liquid pressure P, force acting on thepressure receiving surface 250 a of the piston 250 increases and largetension is applied to the strain detecting portion 120A of the opticalfiber 120 to increase the strain. Conversely, when the amount of liquid200 decreases to lower the liquid pressure P, the force acting on thepressure receiving surface 250 a of the piston 250 decreases to reducethe level of strain in the strain detecting portion 120A.

[0098] In measurement, changes in strain in the strain detecting portion120A of the optical fiber 120 and changes in liquid level height aremeasured in advance through experiments and the correlation therebetweenis determined. Then, after a change in strain is measured by means ofthe B-OTDR 160, a liquid level height corresponding to the measuredvalue is read from the correlation, thereby ensuring that the liquidlevel height H of liquid 200 (the distance between the center of thepressure receiving surface 250 a and the liquid surface 200 a) can bemeasured.

[0099] In the present embodiment, the strain in the optical fiber 120 isdetected using the B-OTDR 160 but this type of detection is notlimitative and measurement can be effected using an optical fiber straingauge based on the different principle, for example, an optical fiberstrain gauge using the fiber Bragg grating (hereinafter abbreviated asFRG) method. The FBG method uses a detecting element using an opticalfiber whose core portion has the refractive index that changesperiodically in the fiber axis direction and in the FBG method, of lightrays coming into the detecting element, only a ray of a specifiedwavelength corresponding to the period of a change in refractive index(Bragg wavelength) is selectively reflected at a fiber grating. Whenstrain is applied to the detecting element, the period of the fibergrating changes and as a result, the frequency of reflection lightshifts. The amount of shift changes in proportion to a strain level inthe optical fiber. Accordingly, by determining an amount of change infrequency shift in the Bragg reflection, the strain in the optical fibercan be measured.

[0100] <Load Designing Method>

[0101] When the pressure receiver 150 and optical fiber 120 are immersedin the liquid 200, the liquid pressure P acting on the pressurereceiving surface 250 a causes the piston 250 to move so as to applytension to the strain detecting portion 120A of the optical fiber 120.In case the rigidity of the optical fiber 120 is large, the liquidpressure P cannot compress air in the vessel main body 150A until theair balances with the liquid pressure and a differential pressurebetween liquid pressure P applied to the pressure receiver 15 andinternal pressure therein is applied to the strain detecting portion120A of the optical fiber 120. In this case, the strain in the straindetecting portion 120A personates the same behavior as that in the caseof normal application of load. Strain (ε) generated in the straindetecting portion 120A is given by the following equation (2).

L={(L _(v) /L _(F))/(L _(v) /L _(F)−ε)·P _(0x·P) ₀+(E _(P) ·ε·D _(P) ²/D _(V) ²)}/(ρ·g)  (2)

[0102] where

[0103] L: water depth

[0104] ε: strain due to elongation of optical fiber core

[0105] D_(V): inner diameter of pressure-tight vessel

[0106] L_(v): inside length (distance between inner surface and bottomof the piston) in the pressure-tight vessel under atmospheric pressure

[0107] D_(P): outer diameter of optical fiber core

[0108] L_(F): length of optical fiber core between the fixing membersunder atmospheric pressure

[0109] ρ: density of liquid

[0110] g: acceleration of gravity

[0111] E_(p): equivalent modulus of elasticity of optical fiber core

[0112] P₀: atmospheric pressure

[0113] Referring to FIG. 14, there is illustrated a liquid-level gaugeaccording to a fifth embodiment of the invention. In the fifthembodiment, a support base 130 is suspended in water by means of atension resistant wire 400 and a B-OTDR 160 is installed on a buoyantbody 410 such as a ship floating on a liquid level 200 a. Othercomponents are identical to those in the fourth embodiment anddesignated by identical reference numerals with their descriptionomitted.

[0114] It will be seen that the liquid level height of the liquid 2 canbe measured with the above construction similarly to the foregoingembodiment.

[0115] Turning to FIGS. 15 and 16, a liquid-level gauge according to asixth embodiment of the invention will be described. In the presentembodiment, a pressure receiving member 150B of pressure receiver 150 isconstructed of a bellows 500 provided with a pressure receiving plate510. The bellows 500 is fixed at one end to an open end surface 150 a ofa pressure-tight vessel main body 150A having the form of a cylinderopened at one end, thus hermetically closing an opening 220 of thevessel main body 150A and has its other opening to which the pressurereceiving plate 510 is mounted. The open end surface 150 a of vesselmain body 150A functions as a stopper for limiting movement of thebellows 500. One surface of pressure receiving plate 510 serving as apressure receiving surface is attached with a fixing member 170.

[0116] In the liquid-level gauge constructed as above, when applied witha liquid pressure P, the pressure receiving plate 510 displaces inaccordance with the pressure to compress and deform the bellows 500. Asa result, tension is applied to a strain detecting portion 120A ofoptical fiber 120 to generate strain. Accordingly, by measuring a changein strain by means of the B-OTDR 600, a liquid level height can bemeasured from the correlation between precedently actually measuredliquid level changes and strain changes, similarly to the foregoingembodiments.

[0117] In the liquid-level gauge using the bellows 500, the vessel mainbody 150A can be sealed more steadily than that in the fourth embodimentusing the piston 250.

[0118] Referring to FIG. 17, a liquid-level gauge according to a seventhembodiment of the invention will be described.

[0119] In the present embodiment, three strain detecting portions120A-1, 120A-2 and 120A-3 are formed at intervals of predetermineddistance in a fiber portion of single optical fiber 120 immersed inliquid 200. More specifically, the respective strain detecting portions120A-1, 120A-2 and 120A-3 are spaced apart from each other in adirection of depth of the liquid 200 and arranged substantiallyhorizontally. Then, three fixing members 140 and three pressurereceivers 150 are immersed in the liquid 200 by making thecorrespondence to the strain detecting portions 120A-1, 120A-2 and120A-3, respectively. Each of the strain detecting portions 120A-1,120A-2 and 120A-3 is fixed at one end A to the corresponding fixingmember 140 through a pipe 180 and is fixed at the other end B to afixing member 170 mounted to a pressure receiving member of thecorresponding pressure receiver 150 through a pipe 190.

[0120] The fixing member 140 for fixing one end A of the lowermoststrain detecting portion 120A-1 and the pressure receiver 150 providedwith the fixing member 170 for fixing the other end B of that straindetecting portion are arranged on a support base 130 laid on the bottomof a tank 100. The fixing member 140 for fixing one end A of theintermediate strain detecting portion 120A-2 and the pressure receiver150 provided with the fixing member 170 for fixing the other end of thatstrain detecting portion are arranged on a first shelf 600 projectingfrom the inner wall of the tank 100 through the medium of a support base130. The fixing member 140 for fixing one end of the uppermost straindetecting portion 120A-3 and the pressure receiver 150 provided with thefixing member 170 for fixing the other end of that strain detectingportion are arranged on a second shelf 610 projecting from the innerwall of the tank 100 through the medium of a support base 130. The threepressure receivers 150 provided to the individual strain detectingportions 120A-1, 120A-2 and 120A-3 are quite identical to each other andthe pressure receiver having the piston type pressure receiving member150B shown in FIG. 13 or the pressure receiver having the bellows typepressure receiving member shown in FIG. 16 is used as each pressurereceiver 150.

[0121] In the liquid-level gauge constructed as above, the lowermoststrain detecting portion 120A-1 is in charge of measurement of a lowliquid level corresponding to a liquid level H1 up to the pressurereceiver 150 of intermediate strain detecting portion 120A-2. Theintermediate strain detecting portion 120A-2 is in charge of measurementof an intermediate liquid level corresponding to a liquid level H2 up tothe pressure receiver 150 of uppermost strain detecting portion 120A-3.Further, the uppermost strain detecting portion 120A-3 is in charge ofmeasurement of a high liquid level corresponding to a liquid level H3exceeding the pressure receiver 150 of intermediate strain detectingportion 120A-2.

[0122] In measurement, when the amount of liquid 200 is small and theactual liquid level H is in the range of H1, in other words, only thepressure receiver 150 of lowermost strain detecting portion 120A-1 isimmersed in the liquid 200, a liquid pressure is applied to the pressurereceiver 150 of lowermost strain detecting portion 120A-1 only. As aresult, tension is applied to the strain detecting portion 120A-1 togenerate strain. The strain is measured by the B-OTDR 160 andthereafter, a liquid level height corresponding to the measured value isread from the correlation between precedently measured strain changesand liquid level heights.

[0123] As the amount of liquid 200 increases and the actual liquid levelH comes into the range of H2, the pressure receiver 150 of intermediatestrain detecting portion 120A-2 is immersed in the liquid 200, liquidpressures are applied to the pressure receivers 150 of the lowermoststrain detecting portion 120A-1 and intermediate strain detectingportion 120A-2. As a result, tension is applied to the two straindetecting portions 120A-1 and 120A-2 to generate strain. The lowermoststrain detecting portion 120A-1 is in association with a maximummeasurable liquid level HI and maximum tension is applied to the straindetecting portion 120A-1 to generate maximum strain. Strain in thelowermost strain detecting portion 120A-1 and strain in the intermediatestrain detecting portion 120A-2 are measured by the B-OYDR 160 andthereafter, a liquid level height (H1+H2) corresponding to the measuredvalues is read from the correlation between the precedently measuredstrain changes and liquid level heights. In other words, a liquid levelheight due to the strain in the intermediate strain detecting portion120A-2 is determined and the thus determined liquid level height isadded with the liquid level H1 measured from the strain in the lowermoststrain detecting portion 120A-1 to determined an actual liquid levelheight.

[0124] As the amount of liquid 200 further increases and the actualliquid level H becomes H1+H2+H3, the pressure receiver 150 of uppermoststrain detecting portion 120A-3 is immersed in the liquid 200 andtension is applied to all of the strain detecting portions 120A-1,120A-2 and 120A-3 to generate strain. The lowermost strain detectingportion 120A-1 is in the measurable range of maximum liquid level Hi andthe intermediate strain detecting portion 120A-2 is in the measurablerange of H2, so that they are applied with the maximum tension togenerate the maximum strain. The strain in all of the strain detectingportions 120A-1, 120A-2 and 120A-3 is measured by means of the B-OTDR160 and thereafter, a liquid level height (H1+H2+H3) corresponding tothe measured strain is read from the correlation between precedentlymeasured strain changes and liquid level heights. In other words, theliquid level height (H3) due to strain in the uppermost strain detectingportion 120A-3 is determined and the thus determined liquid level height(H3) is added with the liquid levels H1 and H2 measured by the lowermoststrain detecting portion 120A-1 and intermediate strain detectingportion 120A-2 to determine an actual liquid level height.

[0125] In the construction as above, the ranges of liquid levelsmeasured by the individual strain detecting portions 120A-1, 120A-2 and120A-3 are distinctively set up and hence, the same maximum tension canbe applied to the individual strain detecting portions 120A-1, 120A-2and 120A-3.

[0126] Referring to FIG. 18, a liquid-level gauge according to an eighthembodiment of the invention will be described.

[0127] In the present embodiment, an end part of single optical fiber120 immersed in liquid 200 is suspended vertically along the inner wallof a tank 100 to provide three strain detecting portions 120A-1, 120A-2and 120A-3 at intervals of predetermined space in a direction of depthof the liquid 200. One end A of each of the strain detecting portions120A-1, 120A-2 and 120A-3 is fixed to a fixing member 140, with theother end B fixed to a fixing member 170 attached to a pressurereceiving member of pressure receiver 150. In other words, the presentembodiment differs from the seventh embodiment of FIG. 17 for horizontalarrangement in that the strain detecting portions 120A-1, 120A-2 and120A-3 are arranged vertically.

[0128] With the construction as above, too, it will be clear that theliquid level height can be measured accurately similarly to the seventhembodiment.

[0129] Referring to FIG. 19, a liquid-level gauge according to a ninthembodiment of the invention will be described.

[0130] In the present embodiment, a strain detecting portion 120A of anoptical fiber 120 has one end A connected to a fixing member 170attached to a pressure receiving member of pressure receiver 150 and theother end B fixed to a fixing member 140.

[0131] With the construction as above, too, it will be clear that theliquid level height can be measured accurately similarly to the fourthto sixth embodiments.

[0132] In the foregoing embodiments, measurement of the liquid levelheight of the liquid 200 in the tank 100 has been exemplified but theinvention is not limited to this and may also be used for measurement ofthe level of liquid placed in hydrostatic pressure condition in, forexample, a storing reservoir.

[0133] The present invention is not limited to measurement with waterrepresenting a liquid but may be used for measurement of the liquidlevel height of various kinds of liquids such as oil and medicines.

[0134] In the foregoing embodiments, the end (A or B) of the straindetecting portion 120A is fixed to the pressure receiving member 150Bvia the fixing member 170 but this is not limitative and that end may befixed directly to the pressure receiving member 150B.

[0135] As described above, the barometric liquid level height measuringmethod and liquid level gauge according to the invention is soconstructed as to measure the liquid level of a liquid by detecting, asa change in strain in the optical fiber, the magnitude of force that isapplied to the pressure receiving member of the pressure receiverconcomitantly with a change in liquid pressure, whereby the constructioncan be simplified remarkably and the number of parts can be reduced tofacilitate the maintenance and decrease the occurrence of troubles.Especially, the optical fiber is immune to electromagnetic induction soas to be insensible to the influence of disturbance and therefore canalso be applied to flammable and non-conductive fluid, thus ensuringthat the liquid level of various kinds of liquids can be measuredaccurately.

[0136] Further, since the stopper for limiting the movement of thepressure receiving member to the predetermined range is provided to thepressure receiver, the strain detecting portion of the optical fiber canbe prevented from being broken.

[0137] In addition, since the ranges of liquid levels to be measured canbe set up distinctively by providing a plurality of strain detectingportions, the same maximum tension can be applied to the individualstrain detecting portions regardless of the liquid level height.

What is claimed is:
 1. A method of measuring a liquid level, wherein anoptical fiber connected at its one end portion to force receiving meansmovably arranged in liquid to receive force from the liquid is dipped inthe liquid together with said force receiving means, and a change in theforce acting on said force receiving means when the liquid level changesis detected as a change in strain in said optical fiber by means of anoptical fiber strain gauge connected to the other end of said opticalfiber.
 2. A liquid-level measuring method according to claim 1, whereinthe precedently determined correlation between changes in strain in saidoptical fiber and changes in liquid level of the liquid is consulted onthe basis of the detected value to determine a water level of theliquid.
 3. A liquid-level gauge comprising an optical fiber, forcereceiving means connected to one end portion of said optical fiber andmovably arranged, together with the optical fiber, in liquid to receiveforce from the liquid, and optical fiber strain measuring meansconnected to the other end portion of said optical fiber to detect, as achange in strain in said optical fiber, a change in the force acting onsaid force receiving means when the liquid level of the liquid changes.4. A liquid-level gauge according to claim 3, wherein said optical fiberstrain measuring means is a Brillouin-optical time domain reflector. 5.A method of measuring a liquid level, wherein a float having across-sectional form that is uniform in the height direction and aspecific weight value less than that of liquid is dipped in the liquid,said float is supported by an optical fiber in such a manner that anupper end of said optical fiber constantly protrudes from the liquidlevel, said optical is connected at its upper end to an optical fiberstrain gauge, and a change in buoyancy acting on said float as the waterlevel of the liquid changes is detected as a change in strain in saidoptical by means of said optical fiber strain gauge, thus measuring awater level of the liquid.
 6. A method of measuring a liquid-level,wherein a suspension member having a cross-sectional form that isuniform in the height direction and a specific weight value not lessthan that of the liquid is suspended by an optical fiber so as to bedipped in the liquid in such a manner that an upper end of saidsuspension member constantly protrudes from the liquid level, saidoptical fiber is connected to an optical fiber strain gauge, and achange in buoyancy acting on said suspension member as the water levelof the liquid changes is detected as a change in stain in said opticalfiber by means of said optical fiber strain gauge, thus measuring awater level of the liquid.
 7. A liquid-level gauge comprising a floathaving a cross-sectional form that is uniform in the height directionand a specific weight value less than that of liquid and dipped in theliquid, an optical fiber for supporting said float in such a manner thatan upper end of said float constantly protrudes from the liquid level,and an optical fiber strain gauge for detecting a change in buoyancyacting on said float due to a change in water level of the liquid as achange in strain in said optical fiber.
 8. A liquid-level gaugecomprising a suspension member having a cross-sectional form that isuniform in the height direction and a specific weight value not lessthan that of liquid, an optical fiber for dipping said suspension in theliquid while suspending said suspension member in such a manner that anupper end of said suspension member constantly protrudes from the liquidlevel, and an optical fiber strain gauge for detecting a change inbuoyancy acting on said suspension member due to a change in water levelof the liquid as a change in strain in said optical fiber.
 9. A methodof measuring a liquid level by generating strain in an optical fiber inaccordance with liquid pressure and detecting the strain to measure aheight of liquid level, portions of said optical fiber dipped in liquidare fixed to a fixing member and a pressure receiving member provided ina pressure receiver, respectively, tension is applied to a fiber portionbetween said fixing member and said pressure receiving member togenerate strain in said optical fiber when said pressure receivingmember is displaced by a liquid pressure, and the strain is detected bymeans of an optical fiber strain gauge.
 10. A liquid-level gaugecomprising an elongated optical fiber, a pressure receiver having apressure receiving member displaceable by liquid pressure, a fixingmeans for fixing said optical fiber, and an optical fiber strain gaugefor detecting strain in said optical fiber, portions of said opticalfiber dipped in the liquid being fixed to said pressure receiving memberand said fixing member, respectively, in the liquid and one end of saidoptical fiber being connected to said optical fiber strain gauge.
 11. Aliquid-level gauge according to claim 10, wherein said pressure receiverhas a pressure-tight vessel main body and said pressure receiving memberis a piston movable over an opening of said vessel main body to cover ithermetically.
 12. A liquid-level gauge according to claim 10, whereinsaid pressure receiver has a pressure-tight vessel main body and saidpressure receiving member is a bellows having a pressure receiving plateto hermetically cover an opening of said vessel main body.
 13. Aliquid-level gauge according to claim 10, 11 or 12, wherein saidpressure receiver is provided with a stopper for limiting movement ofsaid pressure receiving member to a predetermined range.
 14. Aliquid-level gauge according to any one of claims 10, 11, 12 and 13,wherein said optical fiber has a plurality of portions spaced apart fromeach other in a direction of depth of the liquid and each fixed by thepressure receiving member and the fixing member.